Selective anodization apparatus and process

ABSTRACT

THIN FILM TANTALUM RESISTORS ARE TREATED BY SELECTIVE ANODIZATION, BY BRINGING AN ELECTROLYTE INTO CONTACT WITH THE RESISTOR TO BE TREATED AND APPLYING A VOLTAGE BETWEEN THE ELECTROLYTE AND THE RESISTOR. THE ELECTROLYTE IS DISPOSED ON A RAISED PORTION OF A BASE PLATE IN THE FORM OF A THIN LIQUID LAYER WHICH IS PREVENTED FROM EXTENDING LATERALLY BEYOND THE RAISED PORTION BY SURFACE TENSION FORCES.

March 1971 w. H. LAZNOVSKY SELECTIVE ANODIZATION APPARATUS AND PROCESSFiled July 19, 1968 W Z /i 0 a n w N I Arron" United States Patent3,573,176 SELECTIVE ANODIZATION APPARATUS AND PROCESS Wilhelm H.Laznovsky, Princeton, N.J., assignor to RCA Corporation Filed July 19,1968, Ser. No. 746,061 Int. C1. C23]: /48; B23p 1/02 U.S. Cl. 204- 4Claims ABSTRACT OF THE DISCLOSURE laterally beyond the raised portion bysurface tension forces.

BACKGROUND OF THE INVENTION The invention herein described was made inthe course of or under a contract or subcontract thereunder with theDepartment of the Army.

This invention relates to the electrolytic treatment of conductivematerial, and more particularly to electrolytic treatment of a selectedsurface region of such material.

The electrolytic treatment of conductive materials in general, andmetals in particular, is in widespread use. The term electrolytictreatment, as employed in this specification, is defined as a processwhich comprises contacting a conductive body with an electrolyte, andapplying a potential difference between the electrolyte and the body toproduce a chemical or physical change in the structure of the body.Electrolytic treatment as herein defined includes electrolyticanodization, electrolytic etching and electroplating.

In electrolytically treating a metal surface, it is often desired to sotreat only a selected portion of the surface, or to treat the surface toestablish a particular pattern of anodization, etching orelectroplating. The techniques most commonly employed for such selectiveelectrolytic treatment involves deposition of a masking layer on thesurface to be treated, and definition of the desired pattern in themasking layer, so that only the portions of the conductive body notprotected by the masking layer are exposed to the action of theelectrolyte.

Such masking techniques are relatively complex, particularly where highresolution is desired. The use of a masking layer often results indeterioration of the underlying conductive material, due to chemicalinteraction with the developing and stripping solutions required whenthe masking layer is shaped by photoprocessing methods.

Selective electrolytic etching techniques are known in which a jet ofelectrolyte is spewed forth from an orifice to impinge upon theconductive surface to be etched. Such chemical milling or jetelectrolytic etching are techniques exemplified by US. Pats. 3,302,907and 3,278,- 411. Another technique heretofore described utilizes anetching pencil with a spongy applicator tip containing the electrolyte,as described, e.g., in US. Pat. 8,346,477.

These prior art techniques, although not requiring masking, provide poorresolution and do not permit the preci sion alignment which is requiredfor processing of, e.g., the microminiature printed and thin filmcircuitry utilized by the electronics industry.

SUMMARY OF THE INVENTION Apparatus is provided for electrolyticallytreating a body having an exposed electrically conductive region, by

3,573,176 Patented Mar. 30, 1971 applying an electrolyte to theconductive region. The apparatus comprises a base member which has atreating surface area for receiving the electrolyte. The apparatusincludes surface tension means for confining the electrolyte to thetreating area, so that the electrolyte extends above the treatingsurface. The apparatus also includes means for positioning the body tobe treated adjacent the treating area, so that the conductive region ofthe body contacts the electrolyte. Means are provided for applying apotential difference between the conductive region to be treated and theelectrolyte.

In the drawing:

FIG. 1 shows an electrolytic anodization apparatus according to thepreferred embodiment of the invention;

'FIG. 2 shows the manner in which the apparatus shown in FIG. 1 isaligned to the region to be anodized.

FIG. 3 shows a portion of the apparatus of FIG. 1;

FIG. 4 shows a portion of the apparatus of FIG. 1, according to analternative embodiment of the invention. and

DETAILED DESCRIPTION In the manufacture of thin film circuitry, ametallic layer comprising, e.g., tantalum or hafnium is evaporated ontoa ceramic substrate. A circuit pattern is defined in the evaporatedlayer, either by employing a suitable evaporation mask or by selectivelyetching the evaporated metallic layer. A portion of the, e.g., tantalumlayer is designed to serve as one or more resistor elements, whileanother portion of the layer may be intended to serve as one electrodeof a capacitor structure.

The dielectric for the capacitor elements is formed by converting aportion of the tantalum film to tantalum oxide by electrolyticanodization. The resistance of the tantalum resistance elements isincreased to the desired value by anodizing each element to be adjusted,by converting part of the tantalum comprising the resistance element tothe insulating oxide.

Suitable apparatus for anodizing such tantalum thin film capacitor orresistor elements, as shown in FIG. 1, comprises an anodizing fixture 1,and a rotatable metallic substrate support 2. Secured to the metallicsubstrate support 2 is a ceramic substrate 3 which is held in place bymetal spring clips 4. A tantalum thin film 5 is disposed on one surfaceof the ceramic substrate 3. The tantalum film 5 has been depositedaccording to an electrical circuit pattern which contains three tantalumthin film resistance elements 6, 7 and 8. Each of the resistanceelements 6 through 8 is electrically connected to a peripheral portionof the tantalum film 5 which is in turn electrically contacted by one ofthe spring clips 4. Thus an electrical connection is provided betweeneach of the resistors 6 and 8 and the metallic support 2.

The support 2 is rotatable about a pivot 9, which is maintained inposition by a fixed bearing member 10.

The rotatable support 2 is maintained at a proper spacing from theanodizing fixture 1 by means of the set screw 11, which is threaded toengage corresponding threads in the base plate 12 of the fixture 1.

The fixture 1 further comprises a Plexiglass container 13 mechanicallysecured to the base plate 12. The container 13 has an internal cavity 14which serves as a reservoir for electrolytic liquid. A suitableelectrolyte such as dilute oxalic acid is disposed in the reservoir 14.The bottom surface of the reservoir 14 comprises a thin web portion 15of the container 13. The web portion 15 is flexible and may be deflectedby a threaded rod 16. The threaded rod 16 engages corresponding threadson a control gear 17, which is in turn geared to a liquid level settinggear 18. Rotation of the setting gear 18 results in vertical movement ofthe rod 16, thus varying the amount of electrolyte forced out of thereservoir 14.

The reservoir 14 is covered by a metallic profile plate 19, which ismechanically secured to the Plexiglas container 13. A rubber O ringdisposed between the profile plate 19 and the Plexiglas container 13prevents undesired leakage of electrolyte from the reservoir 14.

The profile plate 19 has a raised treating surface portion 20, thedimensions of which correspond to the area to be electrolyticallytreated. An aperture 21 in the profile plate 19 communicates with thetreating surface 20 and the reservoir 14, so that upon adjustment of therod 16, the web moves upward to force electrolyte from the reservoir 14onto the treating surface 20, which is hydrophilic with respect to (i.e.Wetted by) the electrolyte. The amount of electrolyte forced from thereservoir 14 through the aperture 21 onto the treating surface iscarefully adjusted, so that the electrolyte covers the surface 20 and isretained thereon by surface tension forces which limit the lateralextension of the liquid electrolyte layer covering the treating surface20.

An electrical contact to the electrolye 21 may be provided by theprofile plate 19, which in this case comprises a suitable metal such ascopper. Alternatively, the profile plate 19 may comprise an insulatorsuch as Plexiglas, in which case electrical contact to the electrolytemay be provided by a suitable conductor (not shown) extending to thereservoir 14 through the side of the container 13.

In the event that the resistance element to be anodized has sufficientresistance so that it can be biased to exhibit a substantial voltagedrop (on the order of 0.5 volt or more), no other electrical connectionto the electrolytic liquid need be provided.

By an alignment technique to be hereafter described, the fixture 1 ismoved (by means of a two-dimensional micro-positioning table, not shown)so that when the support 2 is rotated about the pivot 9, a selectedresistance element 7 is brought into contact with the electrolyte layerdisposed on the raised surface of the profile plate 19. The set screw 11is adjusted so that the resistance element 7 contacts the liquidelectrolyte layer disposed on the raised surface 20, but does notcontact the surface 20 itself. Typically, the distance between theresistance element 7 and the raised surface 20 may be on the order of0.1 to 0.5 millimeter.

Electrolytic anodization of the resistance element 7 is effected byapplying a potential difference between this resistance element and theelectrolyte, by means of a battery 22 and a variable series resistor 23connected between the profile plate 19 and the support 2. The value ofthe resistance element 7 is monitored (by means not shown) during theanodization process, and the anodization is terminated when the desiredresistance value is attained.

In addition to employing the apparatus shown in FIG. 1 for electrolyticanodization, it may also be employed for electrolytic etching of or forelectroplating a suitable metal onto a metallic film. To perform theseprocesses, it is only necessary to provide an electrolyte suitable forthe particular process to be realized, and to properly adjust (i) thepolarity and voltage of the battery 22, and (ii) the electrolytictreatment current by means of the variable resistor 23.

The manner in which the liquid electrolyte layer is retained on andlimited in lateral extent by the raised surface 20 will be more clearlyunderstood from FIG. 2, which shows an enlarged view of the profileplate 19 and the adjacent portion of the reservoir 14.

As shown in FIG. 2, the size of the reservoir 14 is adjusted (by meansof the rod 16 and web 15, as previously described) so that a thin layer24 of liquid electrolyte is disposed on the raised surface 20. Since thesurface 20 is hydrophilic with respect to the electrolyte, the liquidlayer 2 adheres to the surface 20 to form a meniscus 25 which terminatesat the edges of the raised surface 20. The liquid layer 24 thereforeassumes the same lateral dimensions as the raised surface 20.

The height of the meniscus 25 above the surface 20 is typically on theorder of a few tenths of a millimeter. The surface tension forcesbetween the liquid layer 24 and the raised surface 20 limit the lateralextent of the liquid layer 24 to the raised surface, so that no liquidis disposed on the part of the profile plate 19 adjacent the raisedsurface 20. Since the liquid layer 24 therefore remains static, nosubstantial quantity of electrolyte is consumed during the electrolyticanodization process. Furthermore, by suitably shaping the raised surface20, the liquid electrolyte layer 24 may be correspondingly shaped, sothat electrolytic anodization may be performed in accordance with anydesired pattern.

An alternative construction for the profile plate, which limits thelateral extent of the electrolyte to the predetermined area, isillustrated in FIG. 3, in which a metallic profile plate 26 is providedwith a coating 27 which has a low surface free energy, so that thecoating 27 is hydrophobic with respect to the electrolyte. Where theelectrolyte comprises an aqueous solution, the layer 27 may comprise asuitable plastic such as polytetrafluoroethylene.

Due to the hydrophobic nature of the layer 27, the upper portion 28 ofthe liquid electrolyte disposed in the aperture 21 has a convex meniscus29. Surface tension forces between the hydrophobic layer 27 and theelectrolyte 28 limit the meniscus 29 to the dimensions of the aperture21. The size of the reservoir 14 is adjusted (by means of the rod 16 andweb 15, as previously described), so that the convex meniscus 29 extendsslightly above the adjacent surface of the hydrophobic layer 27.

The profile plate 26, shown in FIG. 3, is employed in precisely the samemanner as the profile plate I19, the resistance element to be anodizedbeing brought adjacent to the layer 27 so that the resistance element iscontacted by the liquid electrolyte meniscus 29.

It should be understood that the aperture 21 need not be circular, butmay have an elongated shape corresponding to the length of the raisedportion 21 of the profile plate 19, or may have an elongated straight orcurved shape determinative of the pattern of the convex electrolytemeniscus 29 associated with the profile plate 26.

The manner in which the particular resistance element 7 to be anodizedis aligned with the liquid electrolyte layer 24 disposed on the raisedsurface 20 of the profile plate 19 is illustrated in FIG. 4. Aftersecuring the substrate 3 to the rotatable support 2, the support isrotated to an alignment position shown in phantom view in FIG. 4. A setscrew 30 limits the movement of the support 2, so that in the alignmentposition the tantalum film 5 is situated in a plane substantially normalto the raised surface 20 of the profile plate 19.

A prismatic light splitter 31 is situated along a line bisecting theangle between the plane of the tantalum film 5 (in the alignmentposition) and the plane of the raised surface 20. While viewing thesuperimposed images of the raised surface 20 and the particularresistance element 7 to be anodized (through suitable magnifying optics,not shown), an operator adjusts the horizontal position and orientationof the profile plate 19 so that the images viewed through the lightsplitter are in alignment.

The profile plate 19 is mounted on a suitable twodimensionalmicropositioner (not shown), for horizontal adjustment. When the profileplate 19 has been properly adjusted, the light splitter 31 is moved awayand the support 2 is rotated to its operating position, i.e. so that theresistance element 7 contacts the liquid electrolyte layer 24 disposedon the raised surface 20 of the profile plate 19.

I claim:

1. Apparatus for electrolytically treating a body having an exposedelectrically conductive region, by applying an electrolyte to saidregion, comprising:

a base member having a predetermined treating area adapted to receivesaid electrolyte, said base member also having an aperture in a givensurface thereof, the periphery of said aperture at said given surfacedefining said treating area, the portion of said given surface adjacentsaid aperture being hydrophobic with respect to said electrolyte;

means disposing said electrolyte in said aperture so that saidelectrolyte has a convex meniscus extending above said given surface,said electrolyte being confined within the periphery of said aperture atsaid surface by surface tension forces;

means for positioning said body adjacent said treating area so that saidregion contacts said electrolyte; and

means for applying a potential difference between said region and saidelectrolyte.

2. Apparatus according to claim 1 further comprising a reservoir forsaid electrolyte communicating with said aperture.

3. A process for electrolytically treating a selected portion of anelectrically conductive layer, comprising the steps of:

providing a base member having a given surface, said member having anaperture communicating with a predetermined portion of said surface;

disposing an electrolyte in said aperture, the part of said givensurface adjacent said aperture being hydrophobic with respect to saidelectrolyte, so that said electrolyte has a convex meniscus extendingabove the surrounding part of said given surface and confined to saidaperture by surface tension force;

positioning said layer and said member so that said meniscus contactssaid selected portion of said conductive layer; and

applying a potential difference between said conductive layer and saidelectrolyte to electrolytically treat said selective portion.

4. A process according to claim 3, wherein said positioning stepcomprises:

References Cited UNITED STATES PATENTS 1,773,135 8/1930 Flanzer 204-2242,763,608 9/1956 Pool 204-224 3,117,067 1/1964 Jacobs 204-224 3,361,6621/1968 Sutch 204-15 FOREIGN PATENTS 1,050,007 12/1966 Great Britain204-15 TA-HSUNG TUNG, Primary Examiner T. TUFARIELLO, Assistant ExaminerUS. Cl. X.R.

